Semiconductor processing systems are characterized by extremely clean environments and extremely precise semiconductor wafer movement. Industries place extensive reliance upon high-precision robotic systems to move substrates, such as semiconductor wafers, about the various processing stations within a semiconductor processing system with the requisite precision.
Reliable and efficient operation of such robotic systems depends on precise positioning, alignment, and/or parallelism of the components. Accurate wafer location minimizes the chance that a wafer may accidentally scrape against the walls of a wafer processing system. Accurate wafer location on a process pedestal in a process chamber may be required to optimize the yield of that process. Precise parallelism between surfaces within the semiconductor processing systems is important to ensure minimal substrate sliding or movement during transfer from a robotic end effector to wafer carrier shelves, pre-aligner vacuum chucks, load lock elevator shelves, process chamber transfer pins and/or pedestals. When a wafer slides against a support, particles may be scraped off that cause yield loss. Misplaced or misaligned components, even on the scale of fractions of a millimeter, can impact the cooperation of the various components within the semiconductor processing system, causing reduced product yield and/or quality.
This precise positioning must be achieved in initial manufacture, and must be maintained during system use. Component positioning can be altered because of normal wear, or as a result of procedures for maintenance, repair, alteration, or replacement. Accordingly, it becomes very important to automatically measure and compensate for relatively minute positional variations in the various components of a semiconductor processing system.
In the past, attempts have been made to provide substrate-like sensors in the form of a substrate, such as a wafer, which can be moved through the semiconductor processing system to wirelessly convey information such as substrate inclination and acceleration within the semiconductor system. One particular example of such a system is shown in U.S. Pat. No. 6,244,121 to Reginald Hunter. That system includes an inclinometer that has a cavity that is partially filled with a conductive fluid, such as mercury, and an array of probes disposed vertically in the cavity into the conductive fluid. Additionally, the system of the '121 patent provides an accelerometer that mounts to the support platform and senses the acceleration of the sensor device.
High accuracy accelerometers used for level sensing tend to be relatively expensive and large, most notably in the z-axis, because they contain large moving parts. The utilization of bulky accelerometers, such as bulky electrolytic accelerometers, or large microelectromechanical system (MEMS) accelerometers can provide a high signal-to-noise (S/N) ratio, but demand large vertical z-axis space. Additionally, these accelerometers are generally relatively costly and increase the overall cost of the substrate-like sensor.
Given that a substrate-like sensor must, by virtue of its design, be able to move through a semiconductor processing system in the same way that a substrate does, it is imperative that the substrate-like sensor not exceed the physical envelope allowed for the substrate. Common wafer dimensions and characteristics may be found in the following specification: SEMI M1-0302, “Specification for Polished Monochrystoline Silicon Wafers”, Semiconductor Equipment and Materials International, www.semi.org. The selection of the accelerometer for use with the substrate-like sensor is constrained by the issue of cost as well as the height of the overall accelerometer.